1. Field of the Invention
The present invention relates to a method of making silver halide crystalline fibers capable of transmitting infrared light and to the fiber product made by the process. More specifically, it involves such methods and products which relate to improved AgCl.sub.x Br.sub.1-x fibers (where x is 0 to 1.0).
2. Information Disclosure Statement
Silver halide fibers consisting of silver chloride (AgCl), silver bromide (AgBr), and their solid solutions (AgCl.sub.x Br.sub.1-x) show promising properties for the transmission of middle infrared radiation. The fibers can either be bare core fibers, with gas functioning as the lower refractive index, total reflecting medium or polymer coated fibers or core/clad fibers where the cladding contains more AgCl for total internal reflection. These are described in U.S. Pat. Nos. 4,253,731; 4,381,141; 4,504,298 and 4,552,434 and considerable improvements have been reported over time.
Nevertheless, there are some drawbacks in present silver halide crystalline fibers. One basic disadvantage of both bare core and step-index core/clad silver halide fibers is their tendency to deteriorate optically and mechanically probably due to recrystallization of the grain structure, phase separation and silver colloids formation as well as absorption and diffusion of extrinsic impurities.
Transmission and elasticity have been observed to decrease remarkably when present state-of-the-art fibers are bent repeatedly. This is a result of defect formation linked to the plastic deformation of the granular structure of the fibers. Present core/clad fibers frequently show intermeshing between the core and the clad material during their extrusion and, as a result, bad transmission. This defect arises due to inhomogeneous deformation during the conventional extrusion process. Further, fibers produced by alternative manufacturing methods such as multiple step-deformation by drawing the fiber through a row of dies as in, for example, U.S. Pat. Nos. 4,381,141; 4,504,298 and 4,552,434, show defects such as microvoids and microcracks. It is assumed that these scattering defects result from the stretching stress applied in these manufacturing methods. The core/cladding boundary also shows defects if the process of successive rolling by grooved rolls is applied as suggested in some of the above prior art, probably due to the non-symmetric deformation mechanism. Additionally, trace contamination by lubricants or other extrinsic impurities is generally observed when multiple deformation methods are employed and these methods are more complicated and less productive than the conventional extrusion process.